A bit and byte synchronized data communication network has been proposed in accordance with standards specified by the International Telegraph and Telephone Consultative Committee (CCITT) and is described in the "CCITT Sixth Plenary Assembly - Orange Book, Vol. VIII.2, Public Data Networks", published in 1977 by the International Telecommunication Union, Geneva. Various requirements, in particular those designated X.21, X.24, and X.27, promulgated in this CCITT specification call for generation within the data network of bit and byte timing signals which are used for establishing a byte control protocol (BCP) or a bit oriented protocol (BOP) for organizing and handling data messages.
In designing and servicing equipment adapted to interface with this data communication network, it is necessary to have a means for testing the equipment under transmit and receive conditions duplicating those which occur during actual operation of the system. At the same time, it is necessary to be able to test various forms of communication devices in a variety of operational modes. Furthermore, the testing apparatus should be simple to operate, compact, and reliable to facilitate operation at different locations at which testing may be required.